One would have to go a loooong way to genuinely improve on the OEM system cat-back, in short any perf gains there are Brochure Babble :-)
A full 3" system (cat included) 'can' give perf gains but recently even that has proven to be not always true.
The OEM system, when purchased from Eeuroparts.com or similar is quite reasonably priced.. free shipping too, just border fee nonsense.